User devices, such as mobile phones, personal data assistants or tablet computers, may be required to know their position, for example, to operate navigation software and/or other location based services. Such user devices may comprise satellite position system (e.g. Global Positioning System (GPS)) receivers which are operable to receive data from a satellite which allows them to identify their position at any given time. However, satellite positioning systems typically require a line of sight between the satellite and the satellite positioning system receiver. When such a line of sight is unavailable, the user device may not be able to determine its position via satellite positioning.
In order to overcome this limitation of satellite positioning technology, a user device may be provided with alternative means for estimating its position, typically for use when satellite positioning is unavailable or inaccurate. For example, a user device may be capable of estimating its position using data measured from base stations of a cellular telephone network (e.g. 2G, 3G, 3.5G or 4G mobile communications network) together with a triangulation algorithm. Additionally or alternatively, a user device may be capable of estimating its position using data measured from a number of Wi-Fi access points together with data identifying the position of a plurality of said access points. Data identifying the absolute position of access points typically takes the form of a “map” of access point locations. The memory requirements of such a map may be prohibitively large for some user devices.
In order to avoid having to store large maps on the user device, it may be preferable for a user device to communicate with a central server to obtain the minimum amount of local positioning data required for determining its position at any given time. Alternatively, the user device may transmit data measured from the Wi-Fi access points it can “see” (i.e. within range of the user device) at any given time to a central server. In this case, the central server may have a memory (or access to a database) containing the positions of the Wi-Fi access points (or best estimates thereof), enabling it to determine the position of the user device when it has received the data measured from the access points. The central server may then transmit the determined position to the user device. This alternative technique also limits the amount of computation required at the user device to obtain an estimate of its position, saving battery power.
A disadvantage of employing a central server is that a working data communication channel must be available between the user device and the server for the positioning system to work.
Commonly, in areas (e.g. tunnels, shopping centres) where a user device cannot obtain access to a data communications network, the line of sight required to enable satellite positioning signal reception may also be unavailable. Accordingly, in these areas, neither positioning system may be operable. It is therefore desirable to develop a new positioning system which would allow a user device to obtain an accurate estimate of its position under these circumstances.